With the growth and increasing longevity of the population of older Americans, providers of health care and social services will encounter increasing numbers of persons whose functioning is impaired. Development of interventions that are effective and practical in dealing with these limitations requires thorough understanding of their determinants. Although the increasing prevalence of chronic disease with age is certain to be a major factor, prior research suggests that neither disease nor age completely determines how well the elderly function in everyday activities or their general well-being. Social and psychological factors may play an important role, but how well these explain age-related differences in functional status and well-being is not will understood. The reason is that studies of explanatory social and psychological factors have not adequately controlled for the presence of chronic medical and psychiatric conditions, or for differences in their severity. We propose to analyze data being collected for an ongoing investigation, the Medical Outcomes Study (MOS), to explore the reasons for individual variations in health status among elderly and middle-aged adults with one or more chronic conditions. Four aspects of health status will be studied: clinical status, functioning in everyday activities, general well-being, and mortality. We hypothesize that health status at a point in time and the probability of transitions to better or worse states over one and two-year intervals are affected by identifiable and measureable social circumstances and psychological factors. Multiple regression methods will be used to test this hypothesis while controlling for sociodemographic and economic characteristics, personal health practices, and medical condition (diagnosis, disease severity, and comorbidity) measured at baseline. The MOS has the capability to measure and isolate the principal social and psychological factors affecting outcome because of its strengths: breadth and depth of measures of health outcomes; well-validated measures of major outcome and explanatory variables; repeated measurements over a 2-year period; and thorough baseline assessments of medical conditions. Large cross-sectional (N-22,785) and longitudinal (N-2546) samples are available for analysis. Study participants range in age from 18- 108 years; 75% of the longitudinal sample are 45 and older; 37% are 65 and older.